In material processing methodologies, pattern etching comprises the application of a layer of radiation-sensitive material, such as photo-resist, to an upper surface of a substrate, the formation of a pattern in the layer of radiation-sensitive material using photo-lithography, and the transfer of the pattern formed in the layer of radiation-sensitive material to an underlying thin film on the substrate using an etching process. The patterning of the radiation-sensitive material generally involves exposure of the radiation-sensitive material to a pattern of electromagnetic (EM) radiation using, for example, a photo-lithography system, followed by the removal of the irradiated regions of the radiation-sensitive material (as in the case of positive tone resist), or non-irradiated regions (as in the case of negative tone resist) using a developing solution.
More recently, a double patterning approach has been introduced to allow the patterning of smaller features at a smaller pitch than what is currently possible with standard lithographic techniques. One approach to reduce the feature size is to use standard lithographic pattern and etch techniques on the same substrate twice (i.e., LELE, or Litho/Etch-Litho/Etch), thereby forming larger patterns spaced closely together to achieve a smaller feature size than would be possible by single exposure. During LELE double patterning, the substrate is exposed to a first pattern, the first pattern is developed in the radiation-sensitive material, the first pattern formed in the radiation-sensitive material is transferred to an underlying layer using an etching process, and then this series of steps is repeated for a second pattern.
In another approach, the feature size is reduced by repeating the exposure of a standard lithographic pattern on the same substrate twice followed by etch techniques (i.e., LLE—Litho/Litho/Etch, or LFLE—Litho/Freeze-Litho/Etch), thereby forming larger patterns spaced closely together to achieve a smaller feature size than would be possible by single exposure. During LLE double patterning, the substrate is exposed to a first pattern, the substrate is exposed to a second pattern, the first pattern and the second pattern are developed in the radiation-sensitive material, and the first pattern and the second pattern formed in the radiation-sensitive material are transferred to an underlying layer using an etching process. During LFLE, an intervening developing and freezing step is inserted following the first exposure to “freeze” or cross-link the first pattern in the double pattern.
In yet another approach, the feature size is reduced by performing sidewall image transfer. Therein, a thin film is conformally applied to a lithographic pattern, which acts as a mandrel, and then partially removed to form a sidewall spacer on a sidewall of the mandrel. During the partial removal of the thin film from the patterned mandrel and the substrate, the success of a spacer etch process is determined by measuring, among other things, the following performance metrics: (a) the size of the sidewall spacer footing, (b) the depth of the substrate recess, (c) the amount of sidewall spacer critical dimension (CD) slimming, and (d) the depth of the spacer top recess or facet. Conventional spacer etch processes produce unacceptable results in at least one of these performance metrics.